The mammalian olfactory system has the remarkable ability to detect thousands of odors in the environment, with precise discrimination. As the primary neuronal element in the olfactory system, olfactory receptor neurons are responsible for detecting odor molecules in the environment, transforming chemical information to electrical signals and sending these signals to the brain. In contrast to rapid progress in molecular studies on olfaction, there is still a lack of functional information about peripheral coding mechanisms. Little is known about the odor response spectra of individual receptor neurons expressing a particular receptor gene, the overlap in response spectra between receptor neurons expressing different receptor genes, the spatial distribution of ORNs with distinct response spectra, and the relations between odor stimuli and receptor molecules. The long-term goal of this application is to address these critical issues by using the septal organ as a model system. The septal organ (organ of Masera) is a small patch of olfactory epithelium separated from the main olfactory epithelium by a region of respiratory epithelium, lying bilaterally near the base of the nasal septum at the entrance to the nasopharynx. Compared to the main olfactory epithelium, which contains ~2 million receptor neurons expressing ~1000 receptor genes, the septal organ is a much simpler system with only about 20,000 neurons expressing ~10 receptor genes. The specific aim of this project is to characterize the functional properties of receptor neurons in the septal organ, including signal transduction pathways, odor response spectra of different subtypes of receptor neurons and their spatial relations. The approaches involve making patch clamp recordings directly from the dendritic knobs of receptor neurons in an intact epithelial preparation, monitoring odor response activities of hundreds of neurons simultaneously with calcium imaging, and identifying key elements in signal transduction pathways by immunohistochemical staining. Carrying out this project will greatly enhance our knowledge about olfactory coding in general and provide the first physiological clues to the function of the septal organ, which is still a mystery.